The present invention relates to a process for separating and recovering cobalt from an aqueous solution containing cobalt and other metals such as nickel, zinc, copper, etc. More particularly, the invention relates to a process for separating and recovering cobalt with high purity by a liquid-liquid solvent extraction method from the above-mentioned aqueous solution containing cobalt and other metals such as nickel, zinc, copper, etc. such as obtained from acid leaching of a cobalt-containing ore.
There have recently been developed the techniques for selectively separating and recovering cobalt by applying a liquid-liquid solvent extraction method from an aqueous solution containing cobalt such as obtained from acid leaching of an ore containing cobalt together with nickel and other metal, and this initiated a variety of proposals on means for separation of cobalt. Nonetheless, there is not yet established any advantageous process capable of separating and recovering cobalt with high purity from the above-mentioned aqueous solution containing cobalt and other metals such as nickel, zinc, copper, etc.
We have previously found the excellent availability of mono-2-ethylhexyl ester of 2-ethylhexylphosphonic acid (hereinafter referred to as M2EHP) as an effective extractant for the selective extraction of cobalt and succeeded in moderately selectively extracting and separating cobalt from an aqueous solution containing cobalt through a liquid-liquid extraction process by using an organic solvent containing the above-mentioned M2EHP. However, it was still impossible to avoid transfer of small amounts of metal ions such as zinc and copper ions present in the aqueous solution into the organic solvent phase together with cobalt, so that when cobalt in the organic solvent phase is separated and recovered by conventional back extraction (back extraction of metal is called hereinafter "stripping" of a metal), these metal ions are also stripped along with cobalt, making it practically impossible to separate and recover cobalt with high purity.
As a result of further studies for overcoming the problems, we found out that a very selective stripping of cobalt is made possible by controlling the pH of the stripping system within a predetermined range during the stripping of cobalt from the cobalt-containing organic phase, and that other metal (ions) remained in the organic phase afer the above-mentioned stripping of cobalt can be effectively stripped by the second step of stripping while controlling the pH of the system below a predetermined level. The present invention has been reached on the basis of these findings.
It was also found that other monoalkyl esters of alkylphosphonic acid can as well be used as an extractant in the process of the present invention.
It is therefore an object of the present invention to provide a process for separating and recovering cobalt with high purity from an aqueous solution containing cobalt and other metals such as nickel, zinc, copper, etc.
It is also another object of the present invention to provide an advantageous process which is not only capable of separating and recovering cobalt with high purity from an aqueous solution containing cobalt and other metals such as nickel, zinc, copper, etc. but also allows reuse of the organic solvent used for the extraction of cobalt after purifying such a solvent.